Hi Theo, Ramsey has a very valid point. The notion of recursion is best modeled by its automata representations. It takes a while in most computer science curricula for students mature enough to grasp the mathematics, but you are physicist so I think you could dive on in. It may be a struggle at first, but it is a worthy study. Like many relations between math and physics, the theory of computation often presses against external states not included in simple models.
One can draw out the isNumber method using the rules of a finite state automata. Since most OO languages including Java use a stack to manage recursive calls, the machine is a recursive push down automata. As Ramsey said, the states imposed on that stack by Java's Virtual Machine and runtime is a matter that has to be either acquired by papers or empirical evidence. V/R, Daniel Beatty, Ph.D., IEEE Certified Software Development Professional (CSDP) Computer Scientist Code 474300D 1 Administration Circle. M/S 1109 China Lake, CA 93555 daniel.bea...@navy.mil (760)939-7097 -----Original Message----- From: webobjects-dev-bounces+daniel.beatty=navy....@lists.apple.com [mailto:webobjects-dev-bounces+daniel.beatty=navy....@lists.apple.com] On Behalf Of Ramsey Gurley Sent: Wednesday, November 12, 2014 8:46 AM To: Theodore Petrosky Cc: WebObjects-Dev Subject: Re: little off topic, need help understand a recursive call Learning new things from new languages is fascinating. Trying to apply this knowledge in the language you know doesn’t always go so well though. What happens if you give isNumber a really really long string? Stack overflow :-/ Recursion is not one of Java’s strengths. If you want a really fast isNumber and you’re only looking for chars ‘0’ thru ‘9', have a look at ERXStringUtilities.luhnCheck. Treating your chars as ints is the fastest way I’ve found to determine isNumber. It is orders of magnitude faster than Character.isDigit. On Nov 12, 2014, at 4:32 AM, Theodore Petrosky <tedp...@yahoo.com> wrote: > I am trying like mad to learn more. So I have decided to put time into > Recursive algorithms. I do not want to start a conversation whether or not > one should or should not use recursion in this example. That is a > conversation for another day. I don’t believe that using it here will impact > the app at all. > > As an experiment I am trying to use one in my validation method. Would one of > you great minds take a look at this. It works, but I feel I am jumping > through too many hoops. This is my first attempt so have a little pity: > > > public String validateHeight(String value) throws ValidationException { > value = value.trim(); > > if (!isNumber(value)) { > throw new ValidationException("There can be only > numbers in the Height field! (value was " + value +")"); > } > return value; > } > > private boolean isNumber(String stringToCheck) { > if (stringToCheck.length() == 0) return true; > if (!Character.isDigit(stringToCheck.charAt(0))) return false; > return isNumber(stringToCheck.substring(1, > stringToCheck.length())); > } > > > My desire is to get better at this and learning recursion is very important. > Also, when I look at other people’s code, there are times that there are > recursive calls and I struggle to understand them. _______________________________________________ Do not post admin requests to the list. They will be ignored. Webobjects-dev mailing list (Webobjects-dev@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/webobjects-dev/daniel.beatty%40navy.mil This email sent to daniel.bea...@navy.mil
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